In a typical wireless communication network, wireless devices, also known as wireless communication devices, mobile stations, stations (STA) and/or user equipments (UE), communicate via a Radio access Network (RAN) to one or more core networks (CN). The RAN covers a geographical area which is divided into service areas or cell areas, with each service area or cell area being served by radio network node such as an access node e.g. a Wi-Fi access point or a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. The service area or cell area is a geographical area where radio coverage is provided by the access node. The access node operates on radio frequencies to communicate over an air interface with the wireless devices within range of the access node. The access node communicates over a downlink (DL) to the wireless device and the wireless device communicates over an uplink (UL) to the access node.
A Universal Mobile Telecommunications System (UMTS) is a third generation telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High-Speed Packet Access (HSPA) for communication with user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for present and future generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in UMTS, several access nodes may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural access nodes connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the V Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases, such as 4G and 5G networks. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long-Term Evolution (LTE) radio access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network, E-UTRAN/LTE is a 3GPP radio access technology wherein the access nodes are directly connected to the EPC core network. As such, the Radio Access Network (RAN) of an EPS has an essentially “flat” architecture comprising access nodes connected directly to one or more core networks.
With the emerging 5G technologies, the use of very many transmit- and receive-antenna elements is of great interest as it makes it possible to utilize beamforming, such as transmit-side and receive-side beamforming. Transmit-side beamforming means that the transmitter can amplify the transmitted signals in a selected direction or directions, while suppressing the transmitted signals in other directions. Similarly, on the receive-side, a receiver can amplify signals from a selected direction or directions, while suppressing unwanted signals from other directions.
Beamforming allows the signal to be stronger for an individual connection. On the transmit-side this may be achieved by a concentration of the transmitted power in the desired direction(s), and on the receive-side this may be achieved by an increased receiver sensitivity in the desired direction(s). This beamforming enhances throughput and coverage of the connection. It also allows reducing the interference from unwanted signals, thereby enabling several simultaneous transmissions over multiple individual connections using the same resources in the time-frequency grid, so-called multi-user Multiple Input Multiple Output (MIMO).
Overall requirements for the Next Generation (NG) architecture e.g. TR 23.799 v. 0.5.0, and, more specifically the NG Access Technology, e.g. TR 38.913 v. 0.3.0 will impact the design of the Active Mode Mobility solutions for the New Radio Access Technology (NR), see RP-160671 New SID Proposal: Study on New Radio Access Technology, DoCoMo, compared to the current mobility solution in LTE. Some of these requirements are the need to support network energy efficiency mechanisms, future-proof-ness and the need to support a very wide range of frequencies e.g., up to 100 GHz.
One of the main differences, with respect to LTE, comes from the fact that propagation in frequencies above the ones allocated to LTE is more challenging so that the massive usage of beamforming becomes an essential component of NR. Despite the link budget gains provided by beamforming solutions, reliability of a system purely relying on beamforming and operating in higher frequencies might be challenging, since the coverage might be more sensitive to both time and space variations. As a consequence of that a Signal to Interference plus Noise Ratio (SINR) of a narrow link can drop much quicker than in the case of LTE, see R2-162762, Active Mode Mobility in NR: SINR drops in higher frequencies, Ericsson.
To support Transmit (Tx)-side beamforming at a radio network node, a number of reference signals may be transmitted from the radio network node, whereby the wireless device can measure signal strength or quality of these reference signals and report the measurement results to the radio network node. The radio network node may then use these measurements to decide which beam(s) to use for the one or more wireless devices.
A combination of periodic and scheduled reference signals may be used for this purpose.
The periodic reference signals, typically called beam reference signals (BRS) or Mobility Reference Signals (MRS), are transmitted repeatedly, in time, in a large number of different directions using as many Tx-beams as deemed necessary to cover a service area of the radio network node. As the naming indicates, each BRS represents a unique Tx-beam from that radio network node. This allows a wireless device to measure the BRS when transmitted in different beams, without any special arrangement for that wireless device from the radio network node perspective. The wireless device reports e.g. the received powers for different BRSs, or equivalently different Tx-beams, back to the radio network node.
The scheduled reference signals, called channel-state information reference signals (CSI-RS), are transmitted only when needed for a particular connection. The decision when and how to transmit the CSI-RS is made by the radio network node and the decision is signalled to the involved wireless devices using a so-called measurement grant. When the wireless device receives a measurement grant it measures on a corresponding CSI-RS. The radio network node may choose to transmit CSI-RSs to a wireless device only using beam(s) that are known to be strong for that wireless device, to allow the wireless device to report more detailed information about those beams. Alternatively, the radio network node may choose to transmit CSI-RSs also using beam(s) that are not known to be strong for that wireless device, for instance to enable fast detection of new beam(s) in case the wireless device is moving.
The radio network nodes of a NR network transmit other reference signals as well. For instance, the radio network nodes may transmit so-called demodulation reference signals (DMRS) when transmitting control information or data to a wireless device. Such transmissions are typically made using beam(s) that are known to be strong for that wireless device.
Beamforming introduces a possibility to enhance the signal towards a specific location. This enables better signal to noise ratio towards a specific wireless device.
A specific beamforming towards a specific wireless device is handled per Transmission Time Interval (TTI) where a number of factors and measurements are used to determine how the beamforming should look like. With an increasing number of antenna elements, the number of possible beams that theoretically can be created increases a lot.
When performing mobility measurements, it will be quite difficult to create all or the best of any possible beams that is optimal from a location of the wireless device on which the specific wireless device can perform its measurement evaluation needed for mobility triggers. Instead a reasonable number of static beams are created, upon request or configured, that are supposed to cover a specific service area. An increasing number of static beams improves the possibility to get as accurate measurement as possible towards a specific location, but it also increases energy consumption and increases complexity in the RAN system, due to complex configuration, planning etc. The number of static beams to perform generic mobility measurements on is supposed to be a lot less than the dedicated beams possible to beam-form towards a specific wireless device in any location of the coverage area served by the radio network node.
The difference in characteristics of a mobility measurement beam, called a MRS beam or a static beam, and a fully dedicated beam for data transmission towards a specific wireless device, also referred to as UE dedicated beam or dedicated beam for data transmission, also creates a difference in what is measured on and what the result will be if the mobility action is performed and a dedicated beam for data transmission is created towards the wireless device. This may lead to wrong decisions when to perform a beam switch or not. In FIG. 1 the fully lined beam represents the mobility measurement beam and the dashed line beam represents a dedicated beam for data transmission towards a specific wireless device. Thus, it is shown a difference between the mobility measurement signal strength and the UE dedicated, beam-formed, beam signal strength.
In FIG. 2, there is one wireless device and three radio network nodes, AN1, AN2 and AN3, each radio network node provides a respective mobility measurement beam and each radio network node also has one dedicated beam for data transmission towards the wireless device. Note that the dedicated beam for data transmission normally only exists from the radio network node currently serving the wireless device, due to that this type of beam needs a lot of ongoing measurements and feedback from the wireless device to be shaped correctly, and these measurements and feedback are normally only available to the radio network node currently serving the wireless device with data transmissions. A mobility decision for the wireless device based on measurements on mobility measurement beams may lead to a non optimal decision resulting in a reduced or limited performance of the wireless communication network.